Structure, hydrolysis and diffusion of aqueous vanadium ions from Car-Parrinello molecular dynamics

Abstract

A molecular level understanding of the properties of electroactive vanadium species in aqueous solution is crucial for enhancing the performance of vanadium redox flow batteries (RFB). Here, we employ Car-Parrinello molecular dynamics (CPMD) simulations based on density functional theory to investigate the hydration structures, first hydrolysis reaction and diffusion of aqueous V${}^{2+}$, V${}^{3+}$, VO${}^{2+}$, and VO${}_2^{+}$ ions at 300 K. The results indicate that the first hydration shell of both V${}^{2+}$ and V${}^{3+}$ contains six water molecules, while VO${}^{2+}$ is coordinated to five and VO${}_2^{+}$ to three water ligands. The first acidity constants (p$K_{\mathrm{a}}$) estimated using metadynamics simulations are 2.47, 3.06 and 5.38 for aqueous V${}^{3+}$, VO${}_2^{+}$ and VO${}^{2+}$, respectively, while V${}^{2+}$ is predicted to be a fairly weak acid in aqueous solution with a p$K_{\mathrm{a}}$ value of 6.22. We also show that the presence of chloride ions in the first coordination sphere of the aqueous VO${}_2^{+}$ ion has a significant impact on water hydrolysis leading to a much higher p$K_{\mathrm{a}}$ value of 4.8. This should result in a lower propensity of aqueous VO${}_2^{+}$ for oxide precipitation reaction in agreement with experimental observations for chloride-based electrolyte solutions. The computed diffusion coefficients of vanadium species in water at room temperature are found to increase as V${}^{3+}$ $<$ VO${}_2^{+}$ $<$ VO${}^{2+}$ $<$ V${}^{2+}$ and thus correlate with the simulated hydrolysis constants, namely, the higher the p$K_{\mathrm{a}}$ value, the greater the diffusion coefficient.